The invention relates to an apparatus for the protection of a hydraulic brake system of a road vehicle against an evaporation of the brake fluid contained in the wheel brake cylinders of wheel brakes which can be actuated by static pressure and in the brake line sections connected to the wheel brake cylinder.
The high driving performances of modern road vehicles, in particular of passenger cars in the upper mid-size range and above, the top speeds of which may be 200 k/h and above, give rise to considerable problems in the design of suitable functionally reliable hydraulic brake systems. Since the high kinetic energy values which have to be brought down in braking generate correspondingly high amounts of heat in the brake system, risk of a vapor lock in and the consequent failure of the brake system is increased. This problem is all the more serious when the modern vehicles are aerodynamically designed with low drag coefficients (c.sub.w values), to increase driving performance. This substantially suppresses turbulence underneath the vehicle, worsens brake cooling and increases the risk of an overheating of the brakes and a consequent failure of the brake system.
To counter this risk, following a braking operation, others have passed a cooling brake fluid flow from the reservoir via the brake booster--a tandem main cylinder arrangement--located in its basic position after completion of the braking operation, to the wheel brake cylinders and from these via the brake booster back to the reservoir. The heat absorbed by the brake fluid in the wheel brake cylinders is removed by a heat exchanger. Such a system is shown in German Offenlegungsschrift No. 3,143,267.
Although this apparatus affords a perfectly effective protection against evaporation of brake fluids, it suffers from the disadvantage that the technical expenditure necessary for its realization is considerable. Namely, two jointly operable main brake cylinders are required, each having an outlet pressure space assigned to the same brake circuit of the vehicle. The brake fluid flow circulating in this brake circuit in the basic position of the two main brake cylinders being passed in series from a pump to a follow-up channel of one of these pressure spaces to the wheel brake cylinders of the respective brake circuit, to the pressure outlet of the corresponding pressure space of the other main brake cylinder, via the follow-up channel of the other main brake cylinder and a return line to the heat exchanger and into the reservoir. Irrespective of its favorable functional characteristics, this apparatus is therefore suitable at most for use in special-purpose vehicles, e.g. racing vehicles, in which the production costs do not Play a significant part from aspects of competitiveness. For use in production vehicles, even of the upper price classes, this apparatus would be much too expensive.
The object of the invention is therefore to create an apparatus of simple design and affords a reliable protection against a malfunctioning of the hydraulic service brake system of a road vehicle due to heating.
This object is achieved according to the invention by providing an admission pressure valve in line with the inlet pressure space of a wheel brake cylinder and a counter pressure cylinder connected to the counter pressure space of the wheel brake cylinder to maintain at least 4 bar or its equivalent of pressure in the brake cylinder space even after completion of braking.
According to this, the brake fluid contained in the wheel brake cylinders and the sections of the wheel brake line branches connected to the latter is kept, even after completion of a braking operation, at an admission pressure corresponding to a minimum value P.sub.s, which effects an increase in the boiling point of the brake fluid by an appreciable amount. The risk of an evaporation of the brake fluid after completion of the braking operation is thereby drastically reduced. The invention is based on the realization that, by increasing the pressure of the brake fluids, usually polyglycol ether based, by for example 5 bar above atmospheric pressure (0 bar), an increase in boiling point of approximately 70.degree. C. can be achieved. This represents a considerable increase compared with the pertinent value for the boiling point of such brake fluids at atmospheric pressure of approximately 170.degree. C., and produces adequate protection against an evaporation of the brake fluid in all braking situations relevant in practice.
The design of the wheel brake cylinders as stepped cylinders with counterpressure spaces, likewise kept at a constant admission pressure when the brake system is not operated, has the effect of achieving the compensation of the forces acting on the wheel brake cylinder pistons, necessary for the non-activated state of the brake system. Although this consequently requires that the wheel brake cylinders are designed differently from usual, this only involves a relatively small extra expenditure and only results in insignificant extra costs in mass production.
The hydraulic components required for the maintenance and limitation of the admission pressure in the inlet pressure spaces of the wheel brake cylinders and for the introduction of the likewise limited counterpressure in the counterpressure spaces may be designed as simply constructed non-return valve arrangements or pressure transformers, which can be actuated by static pressures. These components can be used in conjunction with brake boosters of customary design and can be produced even with relatively low technical expenditure as elements to be designed for comparatively low pressures.
By keeping the admission pressure at a minimum of 4 bar and preferably between 8 and 10 bar, a high functional reliability of the brake system is provided.
The features of a counterpressure cylinder having a spring loaded piston and integral pressure limiting valve, intended for generating the counterpressure which can be introduced into the wheel brake cylinders, affords both the function of a pressure accumulator providing an adequate pressure and also the suitable limitation of the counterpressure to be introduced into the wheel brake cylinders.
The preferred design of said counterpressure cylinder and integral pressure limiting valve has the advantage that it responds very sensitively to the introduction of a braking operation and the response behavior of the brake system is virtually unaffected by the protective apparatus.
The counterpressure cylinder is a simple design of the protective apparatus according to the invention, suitable for brake systems with vacuum brake power assist unit or brake systems with hydraulic brake power assist unit.
The designs specified for the pressure limiting valve or the non-return arrangements intended for pressure limitation or for mintenance of a limited pressure in the counterpressure spaces and the inlet pressure spaces of the wheel brake cylinders make a simple and space-saving integrated configuration possible.